DESCRIPTION: (Applicant's Abstract) At fast chemical synapses, neurotransmitter release is dynamically modified according to the pattern and frequency of presynaptic spikes. Normally, neuronal firing is variable and can reach frequencies of over 100 Hz. Depending upon the type and state of the synapse, neurotransmitter release can either be enhanced or reduced more than ten-fold compared to release evoked by an isolated stimulus. This has a profound impact on the ability of a neuron to influence the firing of its targets. Our primary goal is to determine how synaptic strength is controlled when firing rates are in the physiological range. These studies have important implications for signal processing and drug actions under realistic conditions. We will first study individual mechanisms and then examine how they interact during trains. We will build upon our previous work to answer unresolved questions regarding three prominent forms of short-term synaptic plasticity: facilitation, delayed release and depression. Because presynaptic calcium participates in all of these forms of synaptic plasticity, optical measurements of presynaptic calcium will be an important part of our experimental approach. First, we will study facilitation, an enhancement of the probability of release that lasts for hundreds of milliseconds. We will determine how presynaptic calcium and the initial release probability control the amplitude and time course of facilitation. Second, we will study delayed release, a long-lived (hundreds of milliseconds) component of release that follows the large, brief phasic release of neurotransmitter. We will determine if delayed release becomes increasingly important during trains, which elevate presynaptic calcium. Third, we will study depression, a use-dependent reduction in synaptic strength, by quantifying the calcium-dependence of recovery from depression. Finally, the resulting insights into these three forms of synaptic plasticity will form the basis of our studies of the behavior of synapses during spike trains, when these mechanisms interact to dictate synaptic strength, Experiments will be conducted in rodent cerebellar brain slices. Two classes of synapses will be studied: facilitating synapses between granule cells and either stellate or Purkinje cells, and depressing synapses between climbing fibers and Purkinje cells. By understanding these two very different types of synapses we hope to develop a general framework for understanding use-dependent changes in synaptic strength.